Apparatus for detecting the position of a piston rod

ABSTRACT

A limit switch dog bar is pivotably connected at one end by a ball joint to a piston rod externally of a cylinder which slidably carries the piston rod. The cylinder having positioned thereon, at the end proximate to the ball joint connection, an annular bearing slidably receiving the switch dog bar with the bearing having a spherical surface between it and a fixed sleeve to allow slight inclination of the bar with respect to the axis of the piston rod. Limit switches mounted to the cylinder adjacent the annular bearing lie in the path of movement of dogs fixed to the bar and movable therewith. The switches transmit signals representing the position of the piston rod correlated to positions of the dog operating the limit switches.

United States Patent Yasui et a].

Aug. 5, 1975 APPARATUS FOR DETECTING THE POSITION OF A PISTON ROD Inventors: Koichi Yasui; Yuzo Nakahara; Ken ji Yamano, all of Kawasaki, Japan Fuji Electric Co., Ltd., Kawasaki, Japan Filed: Aug. 22, 1974 Appl. No.: 499,677

Assignee:

Foreign Application Priority Data Aug. 22, I973 Japan .7 48-98504 US. Cl. 200/47; 310/80; 200/153 LA 1m. c1. HOIH 3/16 Field of Search ..200/47, 153 153 153 LA;3l0/80, 83

Primary ExuminerRobert K. Schaefer Assistant E.\'aminerM. Ginsburg Attorney, Agent,- or Firm-Sughrue, Rothwell, Mion, Zinn & Macpeak 5 7] ABSTRACT A limit switch dog bar is pivotably connected at one end by a ball joint to a piston rod externally of a cylinder which slidably carries the piston rod. The cylinder having positioned thereon, at the end proximate to the ball joint connection, an annular bearing slidably receiving the switch dog bar with the bearing having a spherical surface between it and a fixed sleeve to allow slight inclination of the bar with respect to the axis of the piston rod. Limit switches mounted to the cylinder adjacent the annular bearing lie in the path of movement of dogs fixed to the bar and movable therewith. The switches transmit signals representing the position of the piston rod correlated to positions of the dog operating the limit switches.

3 Claims, 3 Drawing Figures PATENTED AUB 51975 moi APPARATUS FOR DETECTING THE POSITION OF A PISTON ROD BACKGROUND OF THE INVENTION 1. Field, Of The Invention g The present invention relates to, an'improvement in an apparatus for detecting the position of a piston rod which slidably moves within. a linear motor cylinder housing the same, and 'more particularly, to an improved arrangement for mounting a limit switch dog bar ,to that slidable piston rod. a

2. Description Of The Prior Art Conventionally, a piston rod or plunger extends externally of a linear motor cylinder and reciprocates axially within the' cylinder regardless of whether such linear motor is of the electrically driven type or of the fluid pressure actuated type. There have been no special provisions for detecting the position of the piston rod particularly where that position is correlated to the internal position of a piston housed by the cylinder. It is true that a certain class of linear motors of the electrically driven type have been equipped with limit switches which limit switches are adapted to transmit signals related to the position of a movable element forming a part thereof. However, this signal merely represents the fact that the piston has traveled excessively in one direction or the other to alert the operator to an accidental overload condition of the electrical linear motor, and therefore it is not intended for the usual control of linear motors or equivalent cylinder means.

The presently employed manner in which linear motors of either the electrical or hydraulic, pneumatic type are controlled falls into two types. In the first type, a'timer is incorporated in such a way'that the timer is started concurrently with the commencement of movement of the linear motor, that is, operation of the cylinder means so that the position of the pistonrod may be represented by the elapsedtime period. Thus, as a predetermined time expires, an assumption is made that a predetermined position has been reached by the linear moving piston rod for effecting subsequent processes. In the second type of measurement, the linear motor cylinder means are provided with a limit switch dog which moves linearlybr axially,'supported by the cylinder itself or by a-ma'chine body'in which the cylinder is'incorporated and the limit switch dog is adapted to be moved synchronously with the piston rod or linearly moving element of the cylinder means. This limit switch dog engages stationary limit switches which are positionedin the path of travel of the limit switch dog as the piston rod or its equivalent moves axially, whereby signals are transmitted from the switches to associated apparatus to inform that apparatus that the piston rod has reached or passed a predetermined position in its stroke. The first type of measurement is-inconvenient in that there is no conviction that the predetermined position has actually been reached by the piston rod or that the piston rod has been stopped at an intermediate'position relative to its stroke. While the second typeof measurement is free of those problems, it is still unacceptable for the reason that the limit switches and 'the limit switch dog or dogs must be attached to the cylinder means only after the completion of the assembly of that hydraulic motor, electrical linear motor, or other device forming the cylinder means which makes the mass production of the cylinder SUMMARY OF THE INVENTION The piston rod detection apparatus of the present invention acts as an improvement for linear motors, whether hydraulic or electric, wherein a longitudinally or axially movable piston rod extends outwardly of a cylinder and a limit switch dog is fixedly coupled to that piston rod and overlies the cylinder and is movable axi-.

ally with the bar and limit switch dog means are fixed to the bar formovement therewith with at least one,

limit switch position within the longitudinal path of movement of the dog for contact actuation thereby, to indicate a predetermined position of the piston rod relative to its linear path of movement with respect tothe cylinder. Within this type of apparatus, the improvement resides in coupling the bar at one end only to the piston rod externally of the cylinder by means of a ball joint and providing an annular bearing which is fixed to the cylinder near that end which is proximate to the ball joint coupling with the switch dog bore being slidably received within the annular bearing and wherein the limit switch means are fixed to the casing in juxtaposition to that annular bearing. Preferably, the bearing has a spherical surface in contact with its support to allow a slight inclination of the bar and bearing with respect to the axis of the piston rod without materially,

affectingthe operation of the detecting apparatus.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of an electrical linear motor equipped with a conventional apparatus for detecting the longitudinal position of the linear motor piston rod.

FIG. 2 is a plan view of an electrical linear motor incorporating the improved means of the present invention for detecting the position of its piston rod.

FIG. 3 is a longitudinal view, partially in section, of the linear motor of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION cate the position of that rod. In FIG. 1, the prior art ar-J rangement is shown in conjunction with a linear motor which may comprise either an electric motor, or a fluid reciprocating motor, in either case a cylinder C which is fixed, has extending therefrom paired piston rods P which are extended from the cylinder C and retracted therein, that is, the rods P move to the right and left of the cylinder C causing plate Q and a rod-like dog bar B to slide axially of cylinder C, the dog bar being supported for parallel longitudinal movement by way of paired bearings R at respective ends of cylinder C. The length I which is the length of the dog bar B is greater than at. least twice the length of the stroke L of the paired piston rods P. Under this arrangement, the dog bar B moves synchronously with the piston rod P, and it is required that the dog barB be maintained parallel to the axis of the piston rod in order to obtain a smooth sliding motion of the dog bar. The manufacture and assembly of the cylinder and the associated piston rod detection apparatus must be achieved in a highly accurate manner which prevents the adaptation of mass production and necessarily leads to an increase in manufacturing cost for this type of prior art apparatus.

As may be seen by reference to FIGS. 2 and 3, these problems are easily overcome by the present invention, which provides for an improvement in the detecting apparatus.-

Referring to FIGS. 2 and 3, the detecting arrangement of the present invention is provided for a linear motor indicated generally at L which includes an elongated cylinder'lO, the linear motor L being of the electric type including an electric drive motor which operates to extend and retract paired piston rods 1 which are supported by way of bearings 20 at the right end of the linear motor, the rods being joined to each other at that end by a thrust coupling 3 which also permits the movable portion of the piston rod assembly to be joined to a load (not shown). It is necessary only to realize that the paired pistons 1 may extend and retract within the cylinder 10 of the linear motor, that is, the rod and the thrust coupling 3 move axially. The extent of such movement and the position of the piston rod is measured or sensed by way of an improved mechanism which involves an annular bearing 2 positioned near the end of the cylinder 10 from which the piston rods I extend. In other words, the bearing member 2 is positioned at the end of the cylinder 10 which is proximate to the coupling point between the limit switch dog bar and the thrust coupling 3. The limit switch dog bar 5 constitutes a rod-like member having a diameter on the order of the opening within the annular bearing member 2 which receives the bar and the outer surface of the bearing 2 facing its support sleeve 18 is spherical, as is the sleeve 18, to permit a slight inclination of the bar 5 with respect to the longitudinal axis of the linear motor L. Sleeve 18 is fixed to cylinder by screws 19. The second element of the improvement resides in the employment of a ball joint 4 for coupling one end only of the limit switch dog bar 5 to the movable piston rods 1. In this respect, with the dog bar 5 overlying the upper surface of cylinder 10, the female member 17 of the ball joint 4 is fixed to the end shaft 5 while the male member 16 received thereby projects vertically upward from the said thrust coupling 3 and terminates in a spherical'surface which is received within the spherical recess of element 17 extending coaxially from rod 5. A pair of cylindrical dogs 6 and 7 are mounted on the bar 5, these dogs being slidable along the length of the bar but being fixedly located by means of set screws 1 1 carried by the dogs but permitting by rotation of the screws the fastening of the dogs 6 and 7 to the bar at desired axial positions on the bar to respective sides of the annular bearing member 2.

As seen in FIG. 2, a pair of limit switches 8 and 9 are located in close proximity to the bearing member 2 fixed to cylinder 10, on respective sides thereof, the limit switches being provided with switch actuators as at 12 and 13 respectively which are positioned in the path of the moving dogs 6 and 7. In this respect, the 'cylindrical dogs 6 and 7 are provided with tapered end faces as at 14 which make contact with the actuators l2 and 13 during the reciprocation of the piston rods 11 to change the state of the switches and provide signals indicative, upon contact by the dogs 6 and 7 with the switch actuators l2 and 13, of predetermined positions of bar 5 and corresponding positions in terms of the piston rods 1. FIGS. 2 and 3 show the piston rods 1 in retracted positions in which dog 7 engages and depresses the actuator 13 of limit switch 9 so as to cause the switch 9 to deliver a signal representing the fact that the piston rods 1 have reached a predetermined retracted position which is at the inner end of their stroke. In a similar manner, the limit switch 8 delivers a signal which represents the fact that the piston rods have reached a predetermined extended position, that is, the outer extremity of their stroke.

The stroke of the piston rods 1 can be conveniently controlled by adjusting the position of dogs 6 and 7.

It is to be noted that the spherical outer surface of the bearing member 2, in combination with-the ball joint 4, allows a slight inclination of bar 5 with respect to the axis of the piston rods so that the bar can move smoothly irrespective of such minor inclination. This means that the requirement for parallel arrangement becomes less critical and the linear vmotor as well as the piston rod position detecting apparatus can .be mass produced. In addition, since the limit switches are positioned near the bearing means, that is, annular bearing 2, the position detection is effected without involving substantial error regardless of the inclination of the bar, the spherical contact surfaces of the annular bearing 2 facing rod 5 and the ball and joint connection at 4 between members 16 and 17 permitting such inclination freely. Regardless of whether the piston rods 1 are extended or retracted, the dogs 6 and 7 when contacting actuators 12 and 13 of respective switches 8 and 9, are closely adjacent to the annular bearing 2 on one side or the other, and thus the positions of the actuators at the moment of contact with switches 8 and 9 are closely controlled, although some misalignment may be permitted due to the nature of the ball joint connection as at 4 and the spherical contact between the outer periphery of the annular bearing and the sleeve 18 which receives the same. Thus, sleeve 18 and annular bearing from a swivel joint closely proximate to the ball joint 4 for permitting a slight inclination of the bar with respect to the longitudinal axis of the linear motor.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a piston rod position detection apparatus for a linear motor wherein apiston rod is extended axially from and retracted axially into one end of a linear motor cylinder, and a limit switch dog bar is fixedly coupled to said piston rod, overlies the cylinder and moves axially with the piston rod, dog means are fixed to said bar for movement therewith, and at least one limit switch is positioned adjacent the bar and in the path of movement of said dog means for contact actuation to indicate the position of said piston rod, the improvement wherein:

said bar is fixedly coupled at one end only to said piston rod by a ball joint,

an annular bearing concentrically receives said bar and is fixed to said cylinder at the end of said cylinder proximate to the ball joint coupling, and

said annular bearing includes means permitting a slight inclination of the bar with respect to the axis ofthe cylinder without impairing its movement longitudinally in synchronism with said piston rod.

2. The apparatus as claimed in claim 1, wherein the inner diameter of said annular bearing is slightly larger than that of said dog bar, the outer periphery of said annular bearing is arcuate, and said annular bearing is fixed to the end of said cylinder by means ofa fixed antive sides thereof. 

1. In a piston rod position detection apparatus for a linear motor wherein a piston rod is extended axially from and retracted axially into one end of a linear motor cylinder, and a limit switch dog bar is fixedly coupled to said piston rod, overlies the cylinder and moves axially with the piston rod, dog means are fixed to said bar for movement therewith, and at least one limit switch is positioned adjacent the bar and in the path of movement of said dog means for contact actuation to indicate the position of said piston rod, the improvement wherein: said bar is fixedly coupled at one end only to said piston rod by a ball joint, an annular bearing concentrically receives said bar and is fixed to said cylinder at the end of said cylinder proximate to the ball joint coupling, and said annular bearing includes means permitting a slight inclination of the bar with respect to the axis of the cylinder without impairing its movement longitudinally in synchronism with said piston rod.
 2. The apparatus as claimed in claim 1, wherein the inner diameter of said annular bearing is slightly larger than that of said dog bar, the outer periphery of said annular bearing is arcuate, and said annular bearing is fixed to the end of said cylinder by means of a fixed annular sleeve having a corresponding internal spherical surface to that of said bearing to permit said slight inclination of the bar and said bearing with respect to the axis of the cylinder.
 3. The apparatus as claimed in claim 1, wherein said dog means comprises dOgs carried by said switch dog bar on opposite sides of said annular bearing and wherein two limit switches are provided and are positioned closely adjacent said annular bearing, on respective sides thereof. 